Windshield wiper systems in which the wiper motor reverses direction in order to oscillate or reciprocate the windshield blades are known in the art. Examples of such systems are disclosed in WIPO Publication No. WO 95/33638, WIPO Publication No. WO 98/07601, U.S. Pat. No. 6,249,098, and US Publication No. 2002/0030458.
Reversing wiper motor systems face a number of technical issues. One of these relates to position-determining means. In order to accurately control the wipers, it is desirable to ascertain the angular position and velocity of the wiper motor and/or associated gears with a relatively high degree of precision. The prior art presents various means for accomplishing this objective. For example, the system disclosed in U.S. Pat. No. 6,249,098 to Miyazaki et al. employs a multipole ring magnet disposed on the motor shaft and a co-operating magneto-resistance rotation sensor which generates a pulse train that can be processed by an electronic control unit to determine the velocity of the motor. This system also employs a mechanically actuated position sensor to determine the angular position of a worm gear coupled between the motor and the wiper arms. More specifically, a rotating conductive plate fixed to the worm gear functions as a movable contact in relationship to a number of stationary contacts accommodated within the casing of the system. This system is not preferred because it employs a mechanically-actuated position detecting means which is prone to fatigue and corrosion.
WO 98/07601 to Ponziani also employs a multipole ring magnet disposed on the motor shaft and a co-operating sensor to generate a pulse train which can be used by an electronic control unit to determine the velocity of the motor. A rotating interrupt fixed to the worm gear mechanically interrupts the pulse train so as to indicate the wiper end of travel or reversing position. The problem with such a system is its lack of flexibility, since the wipe angle will remain fixed and cannot be adjusted on the fly to compensate for environmental conditions such as wind speed, nor can the system be readily implemented in other vehicles which require different wipe angles. As such, this system generally undermines many of the advantages otherwise provided by a reversing wiper motor system.
US 2002/0030458 to Weber et al. uses an incremental transducer for detecting the angular distance traveled by the wiper. This system is more flexible than the Ponziani system, but is deficient, inter alia, for not being able to immediately ascertain upon power-up the position of the wipers. Instead this system must first cycle the wipers. In addition, this system appears to rely on memory to determine if the wipers are in a parked position.
Another problem faced by reversing-motor windshield wiper system relates to control requirements. It is desirable for the control system to quickly respond to control events. At the same time, it is generally desirable to reduce the current drawn by the motor, particularly since the power source is a battery. These requirements place conflicting constraints on the control system.